Delta variant ups herd immunity threshold, 80%+ need antibodies: Study

The Delta variant has meant that close to 80-90% of the population needs to be vaccinated or have had a previous infection (and thus antibodies) in order for a protective wall of population immunity to be strong enough to slow down the virus, researchers from two premier Delhi institute have said, citing serosurveillance data from before the April-May wave of infections in Delhi and the outbreak that happened despite it.

Herd immunity is reached by highly effective vaccines that can stop transmission, but it has been complicated in the case of the coronavirus Sars-Cov-2 due to mutations and waning immunity. The mutations in the Delta variant made it significantly more transmissible -- by some estimates, more than twice when compared to the virus that was first found in Wuhan.

With that virus, which was estimated to have a basic reproduction number (the number of people the virus can spread to on average, also known as R-nought or R0) was between 2 and 3, and the herd immunity threshold was considered by 60-70%. Now, according to a pre-print study by researchers from Maulana Azad Medical College, Institute of Liver and Biliary Sciences, and the Delhi government health department, this threshold may be 80-90%.

The research refers to Delhi’s January round of sero-surveillance to show that despite 56.13% people testing positive for antibodies, there was a massive wave of Covid-19 with over 28,000 cases reported in a single day at its peak in late April.

“The antibody seroprevalence in January showed a more than two-times increase, coinciding with a rapid decline in the test positivity rate and the daily new incident cases suggestive of high population-level immunity. The high seroprevalence through natural infection was insufficient to achieve herd immunity and avert the second wave of the pandemic in Delhi,” the study says.

There is another instance when herd immunity can collapse -- that is when the mutation makes the virus more resistant to immunity from an infection with an older variant. A surge in cases despite high sero-positivity was previously reported from Manaus in Brazil, where the Gamma variant (P.1) that can significantly evade immune was identified.

“Rapid Covid-19 vaccination with the highest possible coverage remains the most feasible means of combating and ending the Covid-19 pandemic,” even though a previous infection provides higher and longer term immunity,” according to the paper.

The researchers also suggest that serial sero-surveys be continued to monitor the waning population levels of antibodies. Genetic sequencing is also needed to keep an eye out for variants that may be more effective in evading immunity, according to the study.

Dr Nandini Sharma, first author of the study and professor of community medicine at Maulana Azad Medical College, said, “In the study we assumed the R0 of the virus to be 2 and showed that to prevent a huge surge in infections, hospitalisations, and deaths we need 80 to 90% of the population to be sero-positive either through infection of vaccination. The data is from January when we did not anticipate a second wave due to delta whose R0 is much more.”

She said, “Now, we are unlikely to have an outbreak of delta. But the cases might go up if a new variant emerges that is more infectious and can evade the immunity.”

There is a need to continue following Covid-19 appropriate behaviour and preventing large gatherings for the time being, she said.

Dr Ekta Gupta, professor of virology at the Institute of Liver and Biliary Sciences said, “The number of cases have gone down in Delhi because of the high levels of exposure in April and May along with a good pace of vaccination. This, despite people not following Covid-19 appropriate behaviour – this is clear from the fact that we are seeing other respiratory diseases like flu in Delhi again, which had reduced because of masking.”